有界缓存的状态依赖优化

可阻塞的状态依赖操作的结构

加锁模式不同寻常，因为锁是在操作的执行过程中被释放与重新获取的。构成前提条件的状态变量必须由对象的锁来保护，从而使它们在测试前提条件的同时保持不变。如果前提条件尚未满足，就必须释放锁，以便其他线程可以修改对象的转改，否则，前提条件就永远无法变成真。在再次测试前提条件之前，必须重新获得锁。

acquire lock on object state
while(precondition does not hold){
    release lock
    wait until precondition might hold
    optionally fail interrupted or timeout expires
    reacquire lock
}
perform action
    release lock

有界缓存提供的put和take操作中都包含一个前提条件：不能从空缓存中获取元素，也不能将元素放入已满的缓存中。

有界缓存实现的基类

@ThreadSafe
public abstract class BaseBoundedBuffer <V> {
    @GuardedBy("this") private final V[] buf;
    @GuardedBy("this") private int tail;
    @GuardedBy("this") private int head;
    @GuardedBy("this") private int count;

    protected BaseBoundedBuffer(int capacity) {
        this.buf = (V[]) new Object[capacity];
    }

    protected synchronized final void doPut(V v) {
        buf[tail] = v;
        if (++tail == buf.length)
            tail = 0;
        ++count;
    }

    protected synchronized final V doTake() {
        V v = buf[head];
        buf[head] = null;
        if (++head == buf.length)
            head = 0;
        --count;
        return v;
    }

    public synchronized final boolean isFull() {
        return count == buf.length;
    }

    public synchronized final boolean isEmpty() {
        return count == 0;
    }
}

将前提条件的失败传递给调用者

@ThreadSafe
public class GrumpyBoundedBuffer <V> extends BaseBoundedBuffer<V> {
    public GrumpyBoundedBuffer() {
        this(100);
    }

    public GrumpyBoundedBuffer(int size) {
        super(size);
    }

    public synchronized void put(V v) throws BufferFullException {
        if (isFull())
            throw new BufferFullException();
        doPut(v);
    }

    public synchronized V take() throws BufferEmptyException {
        if (isEmpty())
            throw new BufferEmptyException();
        return doTake();
    }
}

class ExampleUsage {
    private GrumpyBoundedBuffer<String> buffer;
    int SLEEP_GRANULARITY = 50;

    void useBuffer() throws InterruptedException {
        while (true) {
            try {
                String item = buffer.take();
                // use item
                break;
            } catch (BufferEmptyException e) {
                Thread.sleep(SLEEP_GRANULARITY);
            }
        }
    }
}

class BufferFullException extends RuntimeException {
}

class BufferEmptyException extends RuntimeException {
}

通过轮询与休眠来实现简单的阻塞

@ThreadSafe
public class SleepyBoundedBuffer <V> extends BaseBoundedBuffer<V> {
    int SLEEP_GRANULARITY = 60;

    public SleepyBoundedBuffer() {
        this(100);
    }

    public SleepyBoundedBuffer(int size) {
        super(size);
    }

    public void put(V v) throws InterruptedException {
        while (true) {
            synchronized (this) {
                if (!isFull()) {
                    doPut(v);
                    return;
                }
            }
            Thread.sleep(SLEEP_GRANULARITY);
        }
    }

    public V take() throws InterruptedException {
        while (true) {
            synchronized (this) {
                if (!isEmpty())
                    return doTake();
            }
            Thread.sleep(SLEEP_GRANULARITY);
        }
    }
}

使用条件队列实现的有界缓存

@ThreadSafe
public class BoundedBuffer <V> extends BaseBoundedBuffer<V> {
    // CONDITION PREDICATE: not-full (!isFull())
    // CONDITION PREDICATE: not-empty (!isEmpty())
    public BoundedBuffer() {
        this(100);
    }

    public BoundedBuffer(int size) {
        super(size);
    }

    // BLOCKS-UNTIL: not-full
    public synchronized void put(V v) throws InterruptedException {
        while (isFull())
            wait();
        doPut(v);
        notifyAll();
    }

    // BLOCKS-UNTIL: not-empty
    public synchronized V take() throws InterruptedException {
        while (isEmpty())
            wait();
        V v = doTake();
        notifyAll();
        return v;
    }

    // BLOCKS-UNTIL: not-full
    // Alternate form of put() using conditional notification
    public synchronized void alternatePut(V v) throws InterruptedException {
        while (isFull())
            wait();
        boolean wasEmpty = isEmpty();
        doPut(v);
        if (wasEmpty)
            notifyAll();
    }
}